Semiconductor laser elements have very advantageous properties, for example, a much reduced surface of light emission, a very intense and very collimated laser beam. Thus, optical systems for laser light can be designed with much shorter focal distances than for systems using less collimated light sources, such as incandescent lamps and light-emitting diodes (LEDs). Thus, the optical systems for laser light have a particularly reduced overall size.
The laser sources used in an automobile generally comprise a semiconductor laser element able to emit an overall monochromatic laser beam of given wavelength and a photoluminescent element able to convert a portion of the monochromatic laser beam into a light beam having a broader wavelength spectrum than that of the laser beam.
However, the use of a semiconductor laser element as such a light source for the light modules of motor vehicles poses certain problems due in particular to the fact that such a light source emits a substantially monochromatic coherent light beam when the photoluminescent element is deteriorated or when it is shifted outside of the path of the laser source. Thus, the type of laser used for lighting or signaling applications on board a motor vehicle emits a laser beam which is liable to pose certain problems of safety in the event of malfunctioning of the photoluminescent element. Such a laser beam is particularly harmful to the eyes of an observer, or at the very least risks blinding a user of the road.
Furthermore, the lighting or signaling functions of motor vehicles require light beams having light with a more extensive spectrum than that of a laser beam, for example, a white light.
In order to solve the problems of safety while transforming the laser beam into luminous radiation adapted for the lighting or signaling functions, it is known to interpose a photoluminescent element in the path of the laser beam. Such a photoluminescent element comprises a photoluminescent substance which is excited by light whose wavelength range includes that of the laser beam, for example blue. Consequently, the photoluminescent element emits light whose wavelength spectrum extends into a wavelength range excluding that of the laser beam, or off-centre with respect to that of the laser beam, for example yellow. Thus, at least a part of the incident light of a given wavelength is converted into light of other wavelengths which emits in all directions.
Moreover, at least another part of the incident light is dispersed by the photoluminescent element. In this way, the dispersed light and the converted light are additively superimposed, for example to form a white light.
For the reasons of safety mentioned above, the photoluminescent element thus takes on particular importance. If the photoluminescent element were to be damaged or removed from the path of the laser beam, for example due to an impact, the concentrated laser beam not converted is liable to be emitted by the light module in the direction initially provided for the exit light beam. In these cases, safety measures need to be considered in order to prevent endangering the users of the road.
One solution contemplated is to place a device for detection of the wavelength of the laser in the path of the laser beam downstream from the photoluminescent element. Thus, when the photoluminescent element is no longer performing its function, the laser beam touches the detection device directly. If such is the case, the power supply of the laser element is interrupted by a means of control of the laser element, such as an electronic control unit.
However, such devices are generally bulky.
Moreover, such devices require a precise arrangement of the detection device. This requires, in particular, a very slight positioning tolerance for the means of guiding the light in relation to the optical means.